It is well known that the excessive generation and sparking discharge of electrostatic charges during the manufacture and use of photographic film and paper products are undesirable. Electrostatic charges are generated on the surfaces of overcoat layers or back side layers in these materials by frictional contact with and separation from dissimilar materials such as transport rollers or even the back side layer with respect to the overcoat of the image forming layers, as in a wound stockroll. Excessive accumulation of charge can cause difficulties in handling and transport during manufacturing or in cameras or photofinishing equipment. Sparking discharges of accumulated charge on film or paper surfaces during manufacturing or subsequent handling can cause unwanted exposure marks in the light-sensitive layers.
Although conductive antistatic agents are often incorporated into the back side of film base or paper base to improve electrostatic charging properties of the film or paper products, the use of these antistatic agent in backing layer alone is usually insufficient for preventing all unwanted electrostatic charges from being generated during manufacturing or in transport in cameras or photofinishing equipment. One of the most widely used methods for preventing the excessive generation of electrostatic charges on photographic film and paper products is to add surface active compounds or surfactants which reduce the amount of charge generated on the surface of the outermost protective layer by frictional contact with and separation from dissimilar surfaces as described above. It is also a common practice to add coating aid surfactants to overcoat layers to improve the layer thickness uniformity of the layers, especially in coating methods for the simultaneous application of two or more layers of solution containing gelatin. The polarity of the static charges formed by frictional contact on the surfaces of most gelatin-containing overcoat compositions which also contain hydrocarbon coating aid surfactants, incorporated for improved coating uniformity during the coating process, is a positive polarity. However, when surfactants containing highly fluorinated alkyl groups in their hydrophobic ends are incorporated into overcoat compositions, the resulting static charging of the overcoat surfaces by frictional contact is reduced in its magnitude of positive polarity or becomes closer to neutral or even negative in polarity. The extent of change in charging behavior depends on the amount of fluorinated surfactant used and its molecular structure, which influences its relative effectiveness in negative charging. The composition and amount of the fluorinated surfactant incorporated in the overcoat layer, in combination with the hydrocarbon coating aid surfactants and other addenda, are selected for optimal performance of the product type under conditions of its manufacture and use. When an effective fluorinated surfactant is used at its optimum amount, the electrostatic charging propensity of the overcoat surface is minimized under those conditions of handling and transport during manufacturing and exposure and processing which are most likely to cause unwanted static charge buildup and static marking.
Not all fluorinated surfactants are equally effective in exhibiting this negative charging property when present in overcoat layer compositions containing hydrocarbon coating aid surfactants and other addenda such as dispersed lubricants. The length of the fluorinated carbon chain and the total number of fluorine atoms and their relative positions on the chain, as well as the composition of other groups in the surfactant molecule, are important factors in influencing the negative charging effectiveness of the surfactant. If the surfactant molecule has one or two —(CF2)nF or —(CF2)nH groups, the minimum number of the value n in for the surfactant molecule to be effective enough in its negative charging property for this application is four if the molecule has one fluorinated group or three if it has two fluorinated groups. If the number of fluorinated carbon atoms in a surfactant molecule with one or two fluorinated carbon chains is too few, the negative charging property is greatly diminished. If the number of fluorinated carbon atoms is too many, the solubility of the surfactant in water solutions is too low to be of practical use.
Babbitt, et al., U.S. Pat. No. 3,850,640, disclosed a combination of surfactants, comprising at least one cationic surfactant and at least one nonionic surfactant, the cationic surfactant having the formula F—(CF2)n—SO2—N(R1)—(CH2)m—N(R2R3R4)+ X−, and the nonionic surfactant having the structure alkylphenoxy poly(hydroxypropylene oxide), used in coating the outermost layer of a multilayer photographic element, providing improved coatability and better control of static electricity on the resulting coated photographic element. Mixtures of anionic surfactant, non-ionic surfactant, and cationic fluorosurfactant are described in Research Disclosure No. 10147 (September 1972), and by Orem, U.S. Pat. No. 5,411,844. Nonionic fluorinated surfactants useful as coating aids and for the control of electrostatic charging in overcoat layers of photographic elements are disclosed in Chen, et al, U.S. Pat. No. 4,582,781. A combination of two surfactants for overcoating both sides of a duplitized black and white X-ray recording material, wherein one of the surfactants is a mixture of Rf—CH2CH2—S—CH(COOH)CHC(═O)HN—CH2CH2CH2N(CH3)2, and Rf—CH2CH2—S—CH(CH2COOH)C(═O)HN—CH2CH2CH2N(CH3)2 is disclosed by Adin, et al., U.S. Pat. No. 6,232,058. A black and white silver halide motion picture sound recording film overcoated with a layer containing a combination of three surfactants, one of which is S-100.
In the past the most readily available fluorinated surfactants which have been especially effective for adjusting static charging properties of photographic film and paper products have been those with mostly perfluoro-octyl groups. Furthermore, most of them are either perfluoro-octyl sulfonate in their original form or have structures that may degrade to a perfluoro-octyl sulfonate compound. Recent reports indicate perfluoro-octyl sulfonate may accumulate in the blood systems of humans and animals and show toxicity in laboratory animals at high chronic levels of ingestion. Therefore there is interest in identifying alternative surfactants which do not exhibit these characteristics. Fluorinated surfactants that do not break down to perfluoro-octyl sulfonate or accumulate less than perfluoro-octyl sulfonate in the blood system of animals are desired. Telomer-formed compounds with CF3(CF2)x—CH2—CH2— groups cannot break down to perfluoro-octyl sulfonate. Quantitative Structure Activity Relationships analyses based on computer software available from SRC (Syracuse Research Corporation) indicate that fluorinated surfactants with telomer-formed fluoroalkyl groups and especially groups which have six or fewer fluorinated carbons (and ethylene groups directed bonded to them) present a lower risk of bioaccumulation.
In addition, fluorinated surfactants used in overcoat layers of photographic elements must have good solubility in the coating solutions of the overcoat layers and provide control of static electric charge, without exhibiting adverse effects on the coating uniformity of the overcoat layer or the underlying image forming layers. An additional requirement is that the surfactants of the protective overcoat layer should not adversely change the photographic performance of underlying image-forming layers.